M. S. Isaacson

The application of reactive ion etching in producing free‐standing microstructures and its effects on low‐temperature electrical transport

We have developed a reliable method of fabricating free‐standing metallic microstructures with dimensions down to 50 nm. This technique is compatible with virtually any metal and can be used to fabricate samples that should be low‐dimensional with respect to both electron and phonon transport. Unlike earlier schemes that effect substrate removal from the front, our technique calls for reactive‐ion etching (RIE) from the reverse side of a Si3N4 membrane substrate, onto which microstructures have been previously patterned by standard methods. By carefully controlling the etching times, the extent of invasive damage of the etch on the microstructures is minimized. We have also performed resistance measurements for etched and unetched Al films on bulk substrates to characterize the effect of etching damage and its consequence on low‐temperature transport. By measuring the low‐field magnetoresistance (MR), the inelastic scattering rate and its dependence on temperature can be determined by fitting data to theo...

An attempt to observe phonon dimensionality crossover effects in the inelastic scattering rate of thin free-standing aluminum films

We compare the inelastic scattering rate in 220-Å-thick free-standing films with nearly identical films on substrates. The scattering rate was determined by fitting the magnetoresistance to theories of quantum transport. Although the films are sufficiently thin so as to modify the three-dimensional spectrum of thermal phonons, we find no significant difference between the scattering rates in free-standing and supported films. While experiments on other materials down to lower temperatures are necessary, we conclude that the cubic temperature dependence of the inelastic scattering rate in these thin aluminum films is not strongly affected by the phonon dimensionality.

Electron heating experiments below the spin glass resistance maximum

We report electron heating measurements below the resistance maximum associated with the spin-glass freezing transition. The differential resistance showed features under the application of a dc electric field which cannot be readily interpreted in a simple electron heating picture. We demonstrate that these anomalies are resolved after computation of the resistance by integration of the differential results at various currents. An experiment was designed and carried out to test this procedure through the use of an applied magnetic field. The integration process is applicable to transport measurements carried out under combinations of ac and dc electric fields.

Modification of aluminum thin films

We have utilized several processing techniques to locally modify the superconducting transition temperature and the normal state resistivity of aluminum thin films. The techniques of ion implantation, application of a magnetic overlayer, and reactive ion etching have been used to fabricate S‐N interfaces with controlled differences in transition temperatures and normal state resistances. Lithographic techniques and CF4 reactive ion etching at low power and pressure reliably produce two‐dimensional S‐N structures of desired dimensions in the limit where the transition temperatures and normal state electronic properties of the two regions are close to each other.

Superconducting‐normal metal interfaces produced by reactive ion etching

We have used CHF3/O2 reactive ion etching to reliably change the superconducting transition temperature (Tc) of aluminum thin films by 10–50 mK. Microanalyses using scanning Auger and scanning electron microscopy (SEM) indicate that the etching process results in a surface layer of high fluorine and low oxygen contents. The analyses show that photolithography and etching can produce very sharp interfaces between etched (lowered Tc) and unetched (higher Tc) regions. This technique is applied to study two specific configurations. First, long strips of aluminum thin films are patterned with a periodic structure of alternating regions of high and low Tc. The resistive transition of these films shows a surprisingly long proximity effect with a length scale of more than 50 μm, nearly two orders of magnitude greater than expected from established theory. Second, an aluminum strip with a single S–N–S (high–low–high Tc ) structure possesses a resistive anomaly just above the lower Tc. This anomaly depends on the p...

Aluminum than films with in-plane modulated superconducting structures☆

Abstract We have investigated superconducting properties of aluminum thin films with periodically modulated regions of different transition temperatures (Tc). These samples are made by microlithography and reactive ion etching techniques. They provide a simple and flexible alternative to metallic superlattices previously used in studying the effects of periodicity and anisotropy on superconductivity. Results indicate that these films show global superconducting behavior with properties determined by modulation length scales.

Processing techniques to locally alter the Tc of aluminum thin films

Abstract We report on the successful use of reactive ion etching techniques and ion implantation to locally alter the superconducting transition temperature of aluminum thin films by as much as 5%. The mechanism by which Tc is shifted is by the replacement of surface oxygen with fluorine in the case of the reactive ion etch, with a secondary effect due to damage being evident at long exposure times. The ion implantation produces a shift in Tc of similar magnitude through the damage induced uniformly in the entire film.

Effect of applied field on the spin glass CuCr

Abstract We have examined the resistance of the spin glass, CuCr, as a function of applied magnetic field. The temperature dependence of the position of the resistance maximum is used to characterize the spin glass freezing. A simple functional dependence of this maximum temperature on applied field is proposed to allow quantitative comparison of different samples in terms of parameters that we introduce.

The Electron-Phonon Scattering Rate in Thin Free-Standing Metallic Films

There exists much controversy in the literature regarding the origin of various power laws manifested in the electron-phonon scattering rate (τ ep −1 ) in metallic films. From theory for bulk systems [1], τ ep −1 varies as T3 or T4 for the clean or dirty limit, with the power decreasing by T1 for systems with a 2D phonon excitation spectrum. Experimentally observed power laws for τ ep −1 range from 1.4 to 3.7. Typically, the lower values have been attributed to phonons whose excitation spectrum is significantly altered. However, there have been few attempts to systematically explore this hypothesis by varying the sizes.